This invention pertains generally to monopulse radars, and particularly to signal processing techniques for use in processing signals in monopulse radars to derive normalized pitch and yaw error signals that define bore-sight angle error of any target.
It is known in the art that a monopulse receiver with a single channel intermediate frequency receiver may be used to determine pitch and yaw error signals by appropriate frequency or time multiplexing of the monopulse sum, pitch and yaw error signals. Thus, if radio frequency (R.F.) monopulse sum, pitch and yaw error signals are downshifted to intermediate frequency (I.F.) signals at different intermediate frequencies within the pass band of a single I.F. channel, then demultiplexed and normalized, the angle error of a target may be derived.
It is also known in the art that a monopulse receiver with a two-channel I.F. receiver may be used with a so-called "dot-product angle-error-detector circuit" to derive angle error signals. Thus, if the R.F. monopulse sum signal is alternately combined the pitch and yaw error signal to produce composite signals (S+D) and (S-D) (where S is the monopulse sum signal and D is either the pitch or the yaw error signal), such composite signals may be downshifted to an appropriate I.F. frequency, passed through a two-channel I.F. amplifier and applied to a dot-product angle error detector circuit, or equivalent, finally to derive the desired angle error of a target.
Although known single channel or two-channel monopulse receivers may be useful in many circumstances, neither is satisfactory in all applications. Thus, the single channel receiver may suffer from cross-coupling between the monopulse pitch and yaw error signals, with the result that a correct determination of the angle error of any target cannot be made. Further, the use of an I.F. amplifier with a wide pass band decreases the signal-to-noise ratio and also renders such an amplifier more susceptible to jamming. With the two-channel I.F. amplifier, satisfactory operation requires that:
(a) amplitude and phase imbalances between the two channels be limited to very low amounts, and
(b) such receiver be used in a benign environment, meaning that jamming signals prevent proper operation of a two-channel I.F. amplifier.
The problems experienced in the use of a two-channel I.F. amplifier are exacerbated when post detection integration (PDI) of the output of a fast Fourier Transform (FFT) is required to increase sensitivity. PDI is accomplished by: (a) processing echo signals in each one of N successive intervals (sometimes referred to as "FFT dwells") to obtain the data required for an N point Fast Fourier Transform for pitch (or yaw); and (b) then deriving the average of M successive FFT dwells. In terms of pulse repetition intervals, the time required to perform PDI, i.e., the length of a PDI dwell, is equal to N.multidot.M pulse repetition intervals. In many practical cases the actual time required to perform a PDI for either pitch or yaw error signals may be in the order of tens of milliseconds. Obviously, then, the actual time required to perform PDI on both pitch error signals and yaw error signals is at least twice the time required to perform PDI on either one of such error signals.
In known two-channel monopulse receivers using PDI, an automatic gain control (AGC) signal is determined just prior to each PDI dwell. An AGC signal so derived is predictive and therefore cannot compensate for unpredictable fluctuations in the amplitude of the sum signal during each PDI dwell. It follows then that a constant angle-error sensitivity may not always be attained.
In addition to the just-discussed difficulty, it is known that gain and phase imbalance between two I.F. amplifiers in a two-channel monopulse receiver may change unpredictably with time and signal level. Therefore, even though the effect of a known amount of gain and phase imbalance could be tolerated (because the resulting fixed bias error in bore-sight could be "calibrated out"), unacceptably large errors due to any such imbalances may be experienced in any monopulse receiver using PDI as described above. It will be noted that, according to the prior art, amplitude and phase imbalances between two I.F. amplifier channels are at least as deleterious to proper operation of a finite impulse response (FIR) filter as to an FFT filter with PDI.
Both a FIR filter and an FFT filter with PDI here are inherently susceptible to improper operation in any environment in which the signal-to-noise ratio (SNR) is low. This is so because, when time-multiplexing is used in processing pitch and yaw error signals, slightly less than half the available signal energy defining either pitch error or yaw error is used. As a result, then, the SNR of the receiver is lowered by some 3 db. Any lowering of the SNR of course increases angle noise associated with any target, making it more difficult to achieve a successful intercept.
Finally, because it may be easy to detect the rate at which any time-multiplexing process between pitch error and yaw error signals is being carried out, any monopulse radar using a two-channel I.F. amplifier is vulnerable to jamming. This is so particularly if the AGC function is synchronized in some fashion to the time-multiplexing process.